The present invention is directed toward alternating current generators. More particularly, the present invention is concerned with high output automotive alternators.
Automotive alternators are known utilizing both field coil and permanent magnet fluxes coupled to a stator coil, hereafter referred to as hybrid alternators. For example, U.S. Pat. Nos. 4,882,515 (hereafter '515) and 4,959,577 (hereafter '577), both to Radomski and both assigned to the assignee of the present invention disclose two such hybrid alternators based upon a Lundell rotor structure having a unitary field coil. Lundell structures are notorious for undesirable flux leakage paths during power generation, thereby resulting in less flux coupling through the stator windings and reduced efficiency. The structure disclosed in '577 reduces such leakage paths and increases stator flux, however it has magnetic flux contribution limitations due to magnet size and retention limitations. The structure disclosed in '515 can successfully accommodate more permanent magnet material into its structure but still exhibits the undesirable flux leakage paths characteristic of Lundell structures. However, hybrid alternators of the variety disclosed in the two patents to Radomski exhibit the desirable characteristic of low stator iron losses due to substantial magnet flux shunting through the rotor structure.
Another variety of hybrid alternator is disclosed in U.S. Pat. No. 5,397,975 (hereafter '975) to Syverson and assigned to Ecoair Corporation. As taught by the disclosure of '975, such a machine has a salient, multi-pole, field coil controlled rotor portion with each pole supporting respective windings of the field coil and a permanent magnet portion in longitudinally spaced relationship to the field coil controlled rotor portion. The permanent magnet portion maintains a permanent magnet flux across an air gap between magnet poles and one portion of a divided stator structure, thereby coupling significant magnet flux through the stator structure. Such a spaced arrangement disadvantageously exhibits significant iron losses due to the substantial magnet flux that is continually coupled through the stator iron. Furthermore, the field coil controlled rotor portion having multiple winding sets characteristically exhibits greater power dissipation than unitary field coil winding configuration such as practiced with Lundell variety rotor structures, and requires each winding set to develop the magneto-motive force (mmf) necessary to drive the flux across the air gap thus requiring greater field currents. Additionally, the salient pole construction of Syverson has some drawbacks relative to coil retention at rotor speeds typically encountered in an automotive application and higher inertia due to the multiple windings.